Option 1: If your project has a root assembly definition (*.asmdef), it needs to reference the packages assembly definition. Details on adding that reference are on https://docs.unity3d.com/6000.0/Documentation/Manual/assembly-definitions-referencing.html#reference-another-assembly

Option 2: If you own the Nakama package and don't want to have users setup the above assembly reference, your packages assembly reference needs to have "Auto Referenced" checked.